You may have an ignition, fuel delivery or compression problem.
1. Check for spark first. If there's no spark, you may have a failed ignition module, ignition pickup, ignition coil or open in the ignition circuit (bad ignition switch or neutral safety switch).
2. If you have spark, check for fuel. On carbureted engines, remove the air cleaner, hold the choke open, look down the carburetor throat and work the throttle linkage. If you don't see any fuel squirting into the carburetor, the problem may be a stuck needle inlet valve in the carburetor, a bad fuel pump, a plugged fuel filter, a plugged or frozen fuel line, an obstructed fuel tank pickup screen, or no fuel (or water contaminated fuel) in your tank.
3. If you have spark and fuel, your timing chain or belt may have broken or slipped. If your engine has a distributor, remove the distributor cap and see if the rotor turns when the engine is cranked. No movement would tell you the timing belt or chain (or possibly the cam itself) is broken. Another alternative is to remove the valve cover to see if the valves are opening and closing. This too will show you if the cam drive or cam is broken.
If a cam belt or chain has "slipped a tooth," throwing valve timing off, the valves will still open and close, and the rotor inside the distributor will still turn. But the engine won't develop enough compression to start. A compression check can help you find this kind of problem.